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virtio-scsi: stop using aio_disable_external() during unplug

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This patch is part of an effort to remove the aio_disable_external()
API because it does not fit in a multi-queue block layer world where
many AioContexts may be submitting requests to the same disk.

The SCSI emulation code is already in good shape to stop using
aio_disable_external(). It was only used by commit 9c5aad84da1c
("virtio-scsi: fixed virtio_scsi_ctx_check failed when detaching scsi
disk") to ensure that virtio_scsi_hotunplug() works while the guest
driver is submitting I/O.

Ensure virtio_scsi_hotunplug() is safe as follows:

1. qdev_simple_device_unplug_cb() -> qdev_unrealize() ->
   device_set_realized() calls qatomic_set(&dev->realized, false) so
   that future scsi_device_get() calls return NULL because they exclude
   SCSIDevices with realized=false.

   That means virtio-scsi will reject new I/O requests to this
   SCSIDevice with VIRTIO_SCSI_S_BAD_TARGET even while
   virtio_scsi_hotunplug() is still executing. We are protected against
   new requests!

2. scsi_qdev_unrealize() already contains a call to
   scsi_device_purge_requests() so that in-flight requests are cancelled
   synchronously. This ensures that no in-flight requests remain once
   qdev_simple_device_unplug_cb() returns.

Thanks to these two conditions we don't need aio_disable_external()
anymore.

Cc: Zhengui Li <lizhengui@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-5-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2023-05-16 15:02:22 -04:00 committed by Kevin Wolf
parent 4382f167cf
commit ca66df878e
1 changed files with 0 additions and 3 deletions
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3
hw/scsi/virtio-scsi.c
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@ -1091,7 +1091,6 @@ static void virtio_scsi_hotunplug(HotplugHandler *hotplug_dev, DeviceState *dev,
VirtIODevice *vdev = VIRTIO_DEVICE(hotplug_dev);
VirtIOSCSI *s = VIRTIO_SCSI(vdev);
SCSIDevice *sd = SCSI_DEVICE(dev);
AioContext *ctx = s->ctx ?: qemu_get_aio_context();
VirtIOSCSIEventInfo info = {
.event = VIRTIO_SCSI_T_TRANSPORT_RESET,
.reason = VIRTIO_SCSI_EVT_RESET_REMOVED,
@ -1101,9 +1100,7 @@ static void virtio_scsi_hotunplug(HotplugHandler *hotplug_dev, DeviceState *dev,
},
};
aio_disable_external(ctx);
qdev_simple_device_unplug_cb(hotplug_dev, dev, errp);
aio_enable_external(ctx);
if (s->ctx) {
virtio_scsi_acquire(s);